1. Technical Field of the Invention
The present invention relates to a flexible wiring board and, more particularly, to a flexible wiring board on which a semiconductor device is placed or is to be placed.
2. Description of the Prior Art
Flexible wiring boards, excellent in flexibility and flexuosity, are in wide use mainly for the wiring of a movable member in the field of electrical equipment and electronic equipment. On the other hand, rigid printed circuit boards (rigid wiring boards) have been used hitherto as wiring boards on which semiconductor devices are placed or are to be placed, while flexible wiring boards on which semiconductor devices are placed or are to be placed have also come into use in recent years.
These flexible wiring boards are in general produced by the following processes. A conductor layer made of conductor, such as copper, is laminated on a base layer of insulating material, such as polyimide, through an adhesive layer if necessary, and then a specific circuit pattern is produced on the conductor layer by a subtractive process and the like. Thereafter, a cover layer of insulating material such as polyimide is further laid over it through an adhesive layer if necessary. In the area of these flexible wiring boards where a semiconductor device is placed or is to be placed, the base layer is provided, on its back surface, with a reinforcing layer for bearing pressure during a wire bonding, and also the cover layer is provided with gold pads for the bonding of the semiconductor devices by the wire bonding.
The gold pads are arranged in position corresponding to terminals of the semiconductor devices. The gold pads are each formed by etching part of the cover layer to expose the conductor layer; nickel-plating the exposed conductor layer to form a surface coating thereon; and then plating the surface coating with gold.
In recent years, with rapid advances in performance and reduction in size, profile and weight of electrical equipment and electronic equipment, it comes to be essential for the flexible wiring board, on which the semiconductor devices are placed or are to be placed, to ensure the reliability of connection with the semiconductor devices placed on board.
It is the object of the present invention to provide a flexible wiring board that can substantially ensure the connection reliability when the board is connected with the semiconductor devices placed on board by the wiring bonding.